1. Field of the Invention
This invention relates to ultra-low noise photon detection in low light conditions and more specifically to an ultra-low noise, high gain interface circuit for single-photon readout of off-the-shelf photodetectors at video frame rates.
2. Description of the Related Art
Optical sensors transform incident radiant signals in the X-ray (.lambda.=0.2 .mu.m) visible (.lambda.=0.4-0.8 .mu.m), near infrared (IR) (.lambda.=0.8-2 .mu.m), shortwave IR (.lambda.=2.0-2.5 .mu.m), mid IR (.lambda.=2.5-5 .mu.m), and long IR (.lambda.=5-20 .mu.m) bands into electrical signals that are used for data collection, processing, and storage such as in real-time digital video signals. Available photodetectors such as photodiodes and photoconductors are inexpensive, exhibit bandwidths that support current video frame rates, are sensitive to wavelengths well into the long IR band, and exhibit a high degree of pixel-to-pixel uniformity when used in an imaging array. However, these photodetectors have no gain, i.e. each incident photon generates a single electron, and thus photodetector imaging systems work very well in moderate to bright light conditions, but generate electrical signals at low light levels that are too small to be read-out by conventional readout circuits.
In low light conditions, the standard photodetector is replaced with an avalanche photodiode that exhibits enough gain so that conventional readout circuits can readout the data at video frame rates with a high signal-to-noise ratio (SNR). The fabrication of avalanche photodiodes is much more difficult and expensive than standard photodetectors because they must exhibit very high controlled gain and very low noise. Furthermore, currently available avalanche photodiodes exhibit relatively poor uniformity, are constrained to much lower wavelengths than standard photodetectors (1.5 .mu.m), and have limited sensitivity due to their relatively low quantum efficiency. Imaging intensified systems use an array of avalanche photodiodes to drive respective display elements such as CCDs or phosphors, and have even lower wavelength capabilities (approximately 0.6 .mu.m max) due to the limitations of the photodiode.
Chamberlain et al. "A Novel Wide Dynamic Range Silicon Photodetector and Linear Imaging Array" IEEE Transactions on Electron Devices, Vol. ED-31, No. 2, February 1984, pp. 175-182 describes a gate modulation technique for single-photon readout of standard photodetectors. Chamberlain provides a high gain current mirror that includes a load FET whose gate is connected to its drain to ensure subthreshold operation and to eliminate threshold voltage V.sub.T non-uniformity. The pixel-to-pixel V.sub.T non-uniformity associated with standard silicon CMOS fabrication processes would otherwise substantially degrade the performance of the imaging array. The signal from the photodetector is injected into the load FET thereby producing a signal voltage at the gate of a gain FET. This signal modulates the gain FET's gate voltage, thereby storing integrated charge in a storage capacitor that is readout and reset via a pair of FET switches.
Although Chamberlain's particular gain modulation technique provides a large dynamic range and is capable of detecting wavelengths into the long IR range, the bandwidth of the current mirror severely restricts the bandwidth of the overall detector. Specifically, the RC time constant seen by the photodetector is the parallel combination of the photodetector's capacitance and the resistance of the load FET. In subthreshold operation, the FET's transconductance is very low and, hence, its load resistance is very large, on the order of 10.sup.14 ohms. As a result, the RC time constant is on the order of seconds. Thus, Chamberlain's gate modulation technique is only practically useful for imaging static scenes such as stars. Furthermore, to achieve large current gain, the load FET is typically quite small. As a result, the load FET exhibits substantial 1/f noise, which under low light conditions seriously degrades the performance of the imaging array.
Kozlowski et al. "SWIR staring FPA Performance at Room Temperature," SPIE Vol. 2746, pp. 93-100, April 1996 describes a phenomenon called "night glow" in the short wavelength infrared (SWIR) band that enables detection on very dark nights where photon flux is on the order of one hundred photons per imaging frame. Kozlowski details InGaAs and HgCdTe detector arrays for use with two different readout circuits. Both use Chamberlain's gate modulation technique but one also buffers the detector node to maintain constant detector bias. Unlike SWIR band and longer wavelength detector arrays, near IR and visible detectors are not sensitive to changes in detector bias, and thus buffering to maintain constant bias is irrelevant.